1. Field of the Invention
The present invention relates to a method for screening early failure of a ceramic capacitor.
2. Description of Related Art
Recently, with the downsizing of various electronic devices, downsizing of electronic parts used in these electronic devices is also strongly requested. Thus, in a capacitor, which is a representative of such electronic parts, a small-sized laminated ceramic capacitor with large capacitance is generally used. In this type of capacitor, the downsizing is carried out by thinning dielectric layers by using a material with high dielectric constant, and a high capacitance is obtained by enlarging a laminated area of the electrodes.
However, in such laminated ceramic capacitors, as explained above, since the dielectric layers are thinned and the laminated area of the inner electrodes is enlarged (the number of laminate layers is increased), the probability of having minute defects in the thin dielectric layers becomes higher, and thus, early failure is apt to occur, which causes lowering of reliability.
Conventionally, in order to screen and eliminate laminated ceramic capacitors which have defects in the dielectric layers, a method has been used wherein the insulation resistance value of each capacitor is measured while applying a rated voltage or a voltage multiplied by several times, at room temperature.
In the laminated ceramic capacitors which have defects in the dielectric layers, some are of a type whose insulation resistance value fluctuates between a nondefective range and a defective range, and some are of a type whose insulation resistance value is deteriorated at a high temperature. However, in the conventional insulation resistance measuring method, since the measurement is carried out at room temperature for a relatively short voltage applying time and since the number of times of measurement is limited to one or two in order to maintain high productivity, it is difficult to detect the type whose insulation resistance value fluctuates when the voltage is applied. Further, it is impossible to detect the type whose insulation resistance is deteriorated in a high temperature.
On the other hand, in order to eliminate capacitors which have defects in the dielectric layers and may cause failure, a method is also used wherein the rated voltage or the voltage multiplied by several times is applied continuously for between several hours and several hundred hours and thereafter the insulation resistance value is measured. However, this method is inferior in mass productivity and raises the cost of the products. Thus, this method can only be used for capacitors of limited use which require especially high reliability.